Exploring Density with Salt and Fresh Water: Par 5

This activity provides an introduction to the interaction of fresh and salt water (estuarine systems). It can also serve as an introduction to a discussion of density. Materials:

For each pair of students:

  • Empty tennis ball container with lid (clear plastic with label peeled off)
  • 1 cup of Morton's water softening salt pellets
  • Golf ball (bright color preferred)
  • Blue food coloring
  • Tap water

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An original activity developed by Windows Team member Dave Mastie
Grade level:
25 minutes
Student Learning Outcomes:
  • Students will be able to explain the density of fresh versus salt water.
  • Students will observe what happens when fresh and salt water come together.
  • Students will explore the solubility of salt in water.
Lesson format:
Hands-on activity and discussion

Standards Addressed:


  1. Place 1 cup of Morton's water softening salt pellets in the empty tennis ball container.
  2. Place golf ball on top of pellets.
  3. Fill tennis ball container with tap water (stop about 2 inches from the top).
  4. Put the lid on the tennis ball container and shake for several minutes (until the golf ball floats). Because the lid for the tennis ball container is not always 100% watertight, you may want to encourage students to place a piece of paper towel over the lid while shaking
  5. Wait a few minutes until the salt water clears.
  6. Slowly pour tap water into the container. To avoid mixing of fresh and salt water, make sure the stream of tap water hits the golf ball as it is poured into the container. The golf ball should sink to a slightly lower position once the tap water is added.
  7. Add 2 drops of blue food coloring to the top of the water in the container. Gently swirl the color in without mixing fresh and salt water.
  8. Observe and describe. The fresh water will stay on top of the salt water. The golf ball will float below the freshwater and above the salt water. The color will stop at the interface of the fresh and salt water to show that they are not mixing.


See assessment ideas in parts 2, 3, and 5 of the background information section below.


Veteran teacher Dave Mastie likes to call this activity Par 5 because it can involve 5 points of discussion:

    1. Density of fresh versus density of salt water
    2. Solubility of salt in water
    3. Estuaries (where fresh and salt water come together)
    4. Color combinations and complementary colors (using dye and a colored ball)
    5. Student designed experiments to explore further

One or more of these points may be appropriate depending on the grade level of your students and the curriculum topics that you are teaching. Each of the five points is discussed in more detail below.

1. Density is the mass per unit volume of a substance (density = mass/volume). Another way to think of density is how closely packed the atoms or molecules are in a given material. The density of water is affected by temperature and salinity. When water temperature increases, water molecules vibrate faster spreading further apart and decreasing the density. Salt water has a higher density than fresh water. A less dense substance will always lie above a more dense substance. In this activity, the colored fresh water lies above the denser salt water. The golf ball is floating on the salt water and would sink through the fresh water.

2. A greater volume of salt can be dissolved in hot water compared to cold water. You may ask your students to devise an experiment to test this.

3. An estuary is an aquatic system where fresh water coming from rivers or streams mixes with salty ocean water. Estuaries are unique ecosystems where life thrives and unique biological interactions occur. The mixing of fresh and salt water is not uniform in an estuary as there are many factors to consider such as currents, weather conditions, and tides. When mixing conditions exist (waves, storms) fresh and salt water mix creating water that is less salty than normal sea water (called brackish water). However, when conditions in an estuary are calm, fresh water from rivers will, for the most part, remain above the salt water from the ocean. This creates a unique situation where fresh water animals are found at the top of the water and marine life is found deeper. Students could be encouraged to devise ways to address these and other factors. For instance, students may create brackish water by shaking their container to mix the fresh and salt water.

4. This activity provides an opportunity to look at complementary colors. Complementary colors are found at opposite sides of the color wheel (examples: red & green, blue & orange, and yellow & purple). If you take a blue golf ball and use orange food coloring, the part of the ball that is elevated in the fresh water will appear black (when one complementary color is viewed through the other, one sees black). If you used a green golf ball and red food coloring, the part of the golf ball elevated in the fresh water would appear black and so on. You can also explore the additive effects of color. For instance, a blue golf ball in yellow coloring will look green. You can actually paint the golf balls (place golf tee through cardboard box cover and then place golf ball on top of tee to paint most easily) with crylon paint to get an assortment of colors for this exercise.

5. There are many extension experiments that could be done to help students answer questions related to density, salinity, and estuaries. For assessment purposes, ask students to formulate a question and then design and implement an experiment to answer their question. For example, students could look at how much salt dissolves in cold water versus hot water, they could experiment to see why different golf balls float at different levels (all American golf balls are the same size, bu their insides are very different!), or they could investigate organisms' reactions to the mixing of fresh and salt water.



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